Very recently intensive investigations were made to provide lithium sensitive electrodes which have a sufficiently high selectivity for lithium ions over sodium ions so that they can be used for the determination of the concentration of lithium ions in such liquid media which contain far higher concentrations of sodium ions than of lithium ions, for instance per mol of lithium ions about one hundred mols of sodium ions. Such high requirements as to the selectivity of lithium over sodium have to be put if the lithium concentration is determined in biological fluids, like blood serum. Corresponding electrodes for the determination of the lithium concentration comprise a lithium selective membrane containing a lithium selective component which has to have a very high selectivity for lithium ions over sodium ions.
With those prior art lithium selective membranes which had the highest selectivity for lithium ions over sodium ions it was possible to determine lithium concentrations in aqueous media containing higher concentrations of sodium ions by using calibration curves which were calibrated with a fixed ion background. The molar lithium concentration in blood serum however is only about one hundredth of the molar sodium concentration in the blood serum and if accordingly the lithium concentration remains constant but the sodium concentration alters then the measured lithium concentration determined according to said fixed ion background calibration method also alters and the determined value of the lithium concentration accordingly is incorrect.
According to the present invention said problem can be avoided by determinating in the sample solution not the absolute lithium concentration or lithium activity but the concentration ratio of lithium ions to sodium ions. The present invention accordingly concerns a new process for the determination of the concentration ratio of lithium ions to sodium ions and an apparatus for performing said process.